This invention relates to cleaning gaseous contaminants from air or from other gases.
Currently, methods for eliminating hazardous contaminants in indoor air use ventilation and/or air-purifiers. Ventilation can be used only where outdoor air is of sufficient quality to be considered harmless. Increased ventilation is not cost-free because the air must be conditioned: heating in the winter season or cooling and dehumidifying in the summer season.
Air-purifiers employing adsorbents to remove gaseous contaminants are readily available from a number of commercial suppliers. Typically, the commercial air-purifier design incorporates an adsorbent bed having adsorbent mass sufficient for six months or longer service period. A blend of various adsorbents necessary to treat the wide spectrum of contaminant species found in commercial and residential applications are often used. One disadvantages of this technology is that regular maintenance is required to replace the adsorbent bed. The use of larger adsorbent beds would increase maintenance intervals, but would also increase the size and cost of the apparatus. It would be most preferable to use smaller beds that require less frequent replacement or that could be regenerated without the need for removal of the bed or apparatus from its installation. A further disadvantage of current adsorbent bed technology is that it does not destroy the contaminant, but simply transfers the contaminant from the air to an adsorbent. The contaminant filled adsorbent, when filled, must be disposed of properly.
Photocatalytic air purifiers for treating indoor air pollution, such as those utilizing a porous bed of photosensitive catalyst, such as titania, exposed to ultraviolet light, represent an alternate approach to adsorbents for removing gaseous contaminants. In contrast to adsorbents, photocatalytic oxidation technology completely destroys gaseous contaminants by oxidizing them to benign products; for example, hydrocarbon contaminants are oxidized to carbon dioxide and water. Unfortunately, most contaminants are present in building air at quite small levels (less than 0.1 parts per million, by volume); and at those levels their rates of oxidation by the photocatalytic process are very slow. The slower the oxidation rate, the larger (and more expensive) the reactor needs to be.
A bed of material that has adsorbed therein certain contaminants from a gas stream is regenerated by heating the bed to release the captured contaminants into a fixed volume of gas that is recirculated through the heated bed and through a photocatalytic gas purifier which oxidizes the released contaminants.
Releasing the contaminants from the adsorbent bed into a fixed volume of gas increases the concentration of contaminants within that fixed volume to a level much higher than the level in the untreated contaminated air or other gas from which the contaminants were originally removed; and the rate of photocatalytic oxidation of those contaminants increases dramatically at such higher concentrations. When a sufficient amount of the contaminants within the fixed volume are destroyed by catalytic oxidation, the adsorbent bed is ready to be reused.
In an exemplary embodiment of the present invention, an adsorbent bed captures the gaseous contaminants from an external contaminated gas stream (such as the air in an office building) passed therethrough. The bed is then taken off-line by halting the flow of the external contaminated gas stream through the bed. The bed is regenerated in situ by heating the bed to release the captured contaminant into a fixed volume of gas that is recirculated through the heated bed and through a photocatalytic gas purifier which oxidizes the released contaminants. Photocatalytic oxidation is rapid due to the high concentration of contaminants in the fixed volume. The regenerated adsorbent bed is then put back on line by again passing the contaminated external gas stream through it.
The method of the present invention takes advantage of the fact that the photocatalytic oxidative process obeys the well-known Langmuir-Hinshelwood kinetics. More particularly and fortuitously, for most gaseous contaminants of concern, the rate of photocatalytic oxidation increases substantially linearly as the concentration of the contaminant increases from initial untreated levels that may be as low as or lower than 0.01 ppm up to levels of 1.0 ppm and often up to about 10.0 ppm. This can mean an increase in the rate of oxidation by a factor of 80 or more for some contaminants if their concentration levels can be increased from, for example, 0.01 ppm to 1.0 ppm. For reasons of economics it is preferred that the concentration be increased by at least a factor of ten and to at least 0.1 parts per million of volume (most preferably to at least 1.0 ppm), and to maintain that increased concentration level until at least 50% and preferably until at least 75% of the contaminants captured by the adsorbent bed is released into the recirculating gas and is photocatalytically oxidized.
During the regeneration step, as the fixed, relatively small volume of gas recirculates through the heated adsorbent bed, contaminants are released from the bed into that volume of gas, and the concentration of contaminants in the recirculating gas increases by a factor of 10 or more, and preferably by a factor of 100 to 1000 or more, as compared to the initial contamination levels in the untreated, external, contaminated gas. The final or highest concentration reached will be determined by a number of factors, including the amount of contaminant adsorbed at initiation of regeneration, the size of the fixed volume, and the temperature to which the adsorbent bed is heated during regeneration.
For most contaminants of interest, oxidation rates increase with increases in the concentration rate, until a plateau oxidation rate is reached. The oxidation rate remains at that plateau as contaminant concentration continues to increase. As this highly contaminated recirculating volume of gas passes through the photocatalytic gas purifier, oxidation occurs at a relatively rapid rate. The rapid oxidation rate continues until the contaminant concentration drops back to low levels. By that time, if parameters such as the size of the fixed volume and the adsorbent bed regeneration temperature were selected properly, the majority of the contaminants, and preferably at least 75% of the contaminants captured by the adsorbent bed will have been destroyed at a relatively rapid rate. For purposes of this invention, a xe2x80x9crelatively rapid ratexe2x80x9d means at least ten times faster than the rate of photocatalytic oxidation that would occur at the initial contaminant concentration levels of the gas stream to be cleaned. This permits the use of smaller and thus less costly photocatalytic gas purifier equipment than would otherwise be required if the external contaminated gas were to be continuously cleaned at low oxidation rates by a single pass through a photocatalytic gas purifier.
Another advantage of the present invention is the elimination of the effort and cost associated with replacing the adsorbent bed, since it is regenerated in situ. Also, because the cleaning period between regenerations may be short, for example daily or weekly, a much smaller adsorbent mass may be used. If adsorbent technology alone were utilized to remove the contaminated gas, the adsorbent mass might need to be large enough to last for many months or even years.
In sum, the present invention uniquely combines two different technologies for removing contaminants from a gas to produce a favorable synergistic effect while minimizing the negative aspects that would arise if those technologies were used independently.
The foregoing and other features and advantages of the present invention will become clear from the following description.